remove -Wcast-align from --enable-warnings

I originally added this warning option based on a misunderstanding of
how it works. it does not warn whenever the destination of the cast
has stricter alignment; it only warns in cases where misaligned
dereference could lead to a fault. thus, it's essentially a no-op for
i386, which had me wrongly believing the code was clean for this
warning level. on other archs, numerous diagnostic messages are
produced, and all of them are false-positives, so it's better just not
to use it.